Trenchers, which are a type of earth excavating machine, come in four basic types: bucket wheel, chain, drum and disk or saw. FIG. 1 illustrates a typical chain type trencher 100, which is intended to represent generally this class of large, powerful machines used for digging deep and/or long trenches. Trenchers of this type are based on a tractor with a propulsion system that most often takes the form of a set of caterpillar tracks or crawlers 102, one on each side of the machine. Other types of propulsion systems can, however, be used. The working element is a digging element that is pivotably mounted to the tractor in a manner that permits it to be lowered into the ground. In this example, the digging element includes a continuous chain 104 mounted with a plurality of spaced-apart cutting teeth 106. The chain travels on a track formed on bar 108. The bar is pivotably connected to the tractor. Hydraulic piston actuator 110 causes the bar to pivot, thus moving its tip up or down to adjust the depth of cut. Details of the chain and its pivotal attachment to the tractor are well known. Mounted to the opposite end of the tractor is power plant 112, typically a diesel-fueled internal combustion engine, as in this example. Controls for operating the crawlers and the digging element are placed in operator cabin 114.
FIG. 2 illustrates the drive train for the excavating element e.g. bucket wheel, chain or disk—of trencher 100 of FIG. 1. The output (not visible) of internal engine 202 is coupled to an input (not visible) of torque converter 204. An output of the torque converter is attached to a rotating drive shaft 206. In this example, rotating drive shaft 206 is coupled with the chain 106 through a chain driven transmission. The drive shaft 206 is coupled to transverse gear box 210, which in turn is coupled through reduction gears 216, by chains 212 and 214, to drive sprocket for 218 that rotates the cutting chain 106.
Not shown in FIG. 2 is a hydraulic system for powering the crawlers. An operator controls movement of the trencher by tuning on and off, and changing the direction of flow, of pressurized hydraulic fluid to the crawlers
These two power systems—the system for powering the digging element described above and the system for powering the crawlers—are represented generically by the schematic of FIG. 3. FIG. 3 represents a machine having two power systems (with either 1 or 2 engines), one for driving its working element and one for driving a locomotive element. System 302 represents a working element powered by a system with power train that includes torque converter 306 or similar fluid on coupling for transmitting rotation from a power source 308 to a working element that imposes a load 310 on the system. System 304 represents a locomotive element of a machine. It includes a power source 312 and a transmission 314 that transmits the power to a locomotive that moves the machine, which imposes a load 316 on the power source. As system 304 moves the machine, an additional load is induced on system 302. Arrow 318 represents this induced load on system 302. In the example of a trencher, operation of the crawler at a higher pull force speed faster than the excavating element digs the trench induces a substantial additional load on the power system for the excavating element. This additional load slows the speed of the excavating element, and increases the pull force of the excavating element resulting in reasonable constant power being delivered by the engine to the excavating element. The decrease in speed of the excavating element can approach stall if the speed decrease is allowed to continue. Prolonged operation of the engine with the excavating element stalled can also lead to over heating of and damage to the torque converter and engine.